(40 second video) Lightbox images are only HD the FIRST time that I open a lightbox, until I refresh the page

TLDR: 40 second Video with annotations (Youtube):

Pages in question: https://www.littleidols.net/gallery

I recently changed my site photo galleries from Stacked to Grid, and enabled lightbox popups.

I am finding that when I click on an image to open its lightbox, the first time that I do this, the lightbox HD image loads as expected.

However when I close the lightbox and click a 2nd image on the page to open a lightbox again, this lightbox HD image does NOT load; instead, the low definition/thumbnail version pops up full screen but does NOT load into the HD version nomatter how long I wait.

40 second Video with annotations (Youtube):

This appears to fix immediately for THAT page when I refresh the page. It remains fixed when returning to the page, at least for some time. It appeared to fix for ALL pages last night when I cleared my browser cache with CCCleaner. I was relieved, but when I visit my site today, the problem is repeating again.

I tried checking that I had Ajax turned off in the Style settings page, however Ajax does NOT appear to turn off when I try to turn it off (images still appear to load fluidly as if Ajax is still enabled). It seems like the setting to disable Ajax does not function?

I also tried deleting all code in the custom CSS box in case something was interfering with the lightbox code...

I have tried it on MS Edge, and Chrome, and Chrome incognito.

I will put a 20 second video clip of the problem I'm having on Youtube.

TLDR: First time I click to open a lightbox on a page, it works. After I close it, the SECOND and subsequent times I try to open a lightbox on that page, the HD image version fails to load and only the SD/thumbnail version popups up full screen and stays like that. After refreshing the page, HD versions pop up in the lightbox as expected from then on. To repeat the issue again, open a different gallery page, open and then close a lightbox there, and then try to open a lightbox once more. It's as though the lightbox function temporarily forgets that it's supposed to load a HD image.

Thank you VERY much for your time in looking into this. Please let me know one way or the other so I can move forward with the site at some point, because at the moment I can't tell users to refresh their browser every time they click on a page. Thank you!!!

40 second Video with annotations (Youtube):